The disclosure relates generally to power plants, and more particularly, to a system for reducing startup emissions in a power plant including a gas turbine.
Gas turbine systems are widely used to generate power. Combined cycle power plants employ a gas turbine system and a steam turbine system to generate power. As gas turbine systems and combined cycle power plants have progressed, power plants using the systems have had increasing operational demands placed upon them. In particular, power plants have been required to remain operational over a larger load spectrum while also meeting environmental regulations. One challenge relating to gas turbine system operation is meeting environmental regulations, e.g., nitrogen dioxide (NOx) and/or carbon monoxide (CO) limitations, during low load operations such as during startup of the system. For example, some environmental regulations require NOx emissions to be as low as 19 kilograms/hour during start up emissions, which is increasingly difficult with larger gas turbine systems. During the start-up of the gas turbine system, a number of operational characteristics create high NOx and CO emissions. For example, in a combined cycle power plant, gas turbine system exhaust may be at about 370° Celsius at startup (approximately 5-20% load) to allow heat recovery steam generator (HRSG) warmup (traditional thermal stress mitigation), mating of steam temperature matching for steam turbine system start, reheat pressure reduction for steam turbine system start (HP turbine section) and gas turbine system fuel heating.
During normal higher load operation, emissions from a gas turbine system are typically controlled by two emission control systems. First, a selective catalytic reduction (SCR) system converts NOx to nitrogen, water and carbon dioxide (CO2) by causing the exhaust to react with a reducing agent, e.g., anhydrous ammonia, aqueous ammonia or urea. Second, the exhaust may be passed through a CO catalyst system to remove CO. However, during low load conditions of a combined cycle power plant, for example, the SCR system and the CO catalyst system are not active because they do not attain the desired operating temperature because they are located after any heat exchanger capable of creating the required heat, e.g., a superheater within the HRSG or a high pressure (HP) drum. For example, at startup it can take more than 30 minutes for the traditional emission control systems to reach sufficient operating temperatures to start reducing NOx and CO emissions. In this case, exhaust exits to atmosphere from the HRSG without emission control. During this initial period, the power plant may continue to emit NOx and CO emissions which are counted against the government issued permit limits for startup and overall yearly tons. This issue can end up putting restrictions on the power plant operability such as limitations on the number of starts and total hours of operation in a year. In order to address CO emissions, additional CO catalysts have been positioned upstream of a superheater, but such structure places further limitations on the power plant during full load operation. In another approach, the load of the gas turbine system is quickly raised from startup to a point where emissions are lower (referred to as ‘rapid response’). However, this approach adds more equipment and complex control systems to the power plant.